IB/umad: Use class_groups and let core create class file

Use class->class_groups core kernel facility to create the abi version
file instead of open coding.

Signed-off-by: Parav Pandit <parav@mellanox.com>
Reviewed-by: Jack Morgenstein <jackm@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>